1. Field of the Invention
The present invention relates to compositions and methods for assaying binding interactions between an analyte of interest and a ligand of interest. More specifically, the invention relates to detecting interactions of analytes and ligands by observing changes in the collective behavior of colloidal particles in solution.
2. Description of the Related Art
Most biomolecules interact with other biomolecules in order to carry out their functions in vivo. For example, cellular processes often involve proteins bound together in multi-subunit complexes. In addition, interactions between multiple types of biomolecules create a diversity of cellular structures, such as the cytoskeleton and cellular membranes. Moreover, many pathogens, diseases, and physiologically significant conditions can be diagnosed by the presence of particular substances within a biological sample. Accordingly, there is a general need for sensitive, low-cost methods of detecting binding between analytes and ligands of interest.
Some ligands naturally reside within cellular membranes. For this reason, cellular membranes have been extensively studied to determine a linkage between those ligands and their functions in vivo. For example, many known therapeutic drugs target biomolecules, such as receptors, that reside on the surface of cellular membranes. A significant challenge in studying biochemical reactions on membrane surfaces is the difficulty in emulating the naturally fluid membrane environment within an in vitro assay. One strategy involves coating solid substrates, such as silica or certain polymers with lipid membranes in order to emulate the structure of cell membranes in vivo (Sackmann, E., Science 271: 43-48 (1996); Groves, J. T., Curr. Op. Drug Disc. & Dev., 5: 606-612 (2002)). Using this technology, membranes were firmly trapped near the solid interface, but also retained their natural fluidity and biological functionality (Grakoui et al., Science 285: 221-227 (1999)).
Lipid membranes floating on a supported surface, such as silica, have been used to study a variety of therapeutically valuable membrane proteins, including G protein-coupled receptors (Fang et al., J. Am. Chem. Soc., 124: 2394-2395 (2002)). However, detection of molecular interactions on those membrane surfaces generally required elaborate techniques such as surface plasmon resonance (SPR) (e.g., Hoffman et al., Proc. Natl. Acad. Sci. USA, 97: 11215-11220 (2000)) or total internal reflection (TIR) microscopy (Yang et al., Anal. Chem., 73:165-169 (2001)). These techniques normally necessitate the use of fluorescent labels.